During the drilling of a wellbore, various fluids are used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through the wellbore to the surface. During this circulation, the drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
As stated above, wellbore fluids are circulated downhole to remove rock, as well as deliver agents to combat the variety of issues described above. Fluid compositions may be water- or oil-based and may comprise weighting agents, surfactants, proppants, viscosifiers, fluid loss additives, and polymers. However, for a wellbore fluid to perform all of its functions and allow wellbore operations to continue, the fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the wellbore fluid may be lost to the formation. For example, wellbore fluid may leave the borehole through large or small fissures or fractures in the formation, or a highly porous rock matrix surrounding the borehole.
One way of protecting the formation is by forming a filtercake on the surface of the subterranean formation. Filtercakes are formed when particles suspended in a wellbore fluid coat and plug the pores in the subterranean formation such that the filtercake prevents or reduce both the loss of fluids into the formation and the influx of fluids present in the formation. A number of ways of forming filtercakes are known in the art, including the use of bridging particles, cuttings created by the drilling process, polymeric additives, and precipitates. Fluid loss pills may also be used where a viscous pill comprising a polymer may be used to reduce the rate of loss of a wellbore fluid to the formation through its viscosity
Upon completion of drilling, the filtercake and/or fluid loss pill may stabilize the wellbore during subsequent completion operations such as placement of a gravel pack in the wellbore. Additionally, during completion operations, when fluid loss is suspected, a fluid loss pill of polymers may be spotted into to reduce or prevent such fluid loss by injection of other completion fluids behind the fluid loss pill to a position within the wellbore which is immediately above a portion of the formation where fluid loss is suspected. Injection of fluids into the wellbore is then stopped, and fluid loss will then move the pill toward the fluid loss location.
After any completion operations have been accomplished, removal of filtercake (formed during drilling and/or completion) remaining on the sidewalls of the wellbore may be necessary. Although filtercake formation and use of fluid loss pills may be used in drilling and completion operations, the barriers may be a significant impediment to the production of hydrocarbon or other fluids from the well if, for example, the rock formation is still plugged by the barrier. Because filtercake is compact, it often adheres strongly to the formation and may not be readily or completely flushed out of the formation by fluid action alone.
To remove filtercakes and fluid loss pills prior to production, breaker fluids are introduced to the wellbore to remove residual filtercake and fluid loss pills. Breaker fluids may contain, for example, solvents, acids, oxidizers, or enzymes, and destroy the integrity of a residual filtercake created during the drilling process by removing some or all fluid components that form the filtercake or fluid loss pill.
As mentioned above, when filtercakes form the particles suspended in a wellbore fluid coat and plug the pores in the subterranean formation such that the filtercake prevents or reduces both the loss of fluids into the formation and the influx of fluids present in the formation. However, weak zones in the formation may be encountered during drilling due to the heterogeneous nature of certain formations. The formation of weak zones during drilling may lead to additional fluid loss, pressures changes, and well cave-ins. Because these weak zones are detrimental to drilling, such zones must be strengthened before drilling work can resume, resulting in increased operation costs. Further, lost circulation of the drilling fluid is a recurring drilling problem, characterized by loss of drilling fluid into downhole formations that are fractured, highly permeable, porous, cavernous, or vugular.
Weak zones within a wellbore may be strengthened, for example, during drilling by chemically modifying the filtercake, to form a chemical casing while drilling or by employing separately applied fluid treatments that are pumped into the weak zone and allowed to cure and develop strength over time. Such treatments have included polymeric composites and cement compositions, which are also used in primary cementing operations, as a lost circulation treatment and/or in zonal isolations.